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Some animals determine sex based on genes - there's multiple approaches to this, including the XY system (most common in mammals); the XO system (many invertebrates) the ZW system (most common in birds); and the XYXYXYXYXY system (pretty much just platypuses and echidnas). Under these systems, if an embryo has one type of chromosome set, it will develop as female; the other and it will develop as male (most of the time).

Anyway, back to frogs. The common frog is unusual in that it has "sex races". Not as in speedrunning a shag. Common frogs have multiple different approaches to sex determination which vary by region.

A common frog living in colder northern climates will probably hatch as either female or male. The male will have XY chromosomes; the female XX. However, a frog born in a milder climate such as the Netherlands or the UK will always hatch with ovaries, regardless of chromosomes.

Once they've metamorphosed from a tadpole into a froglet, some common frogs from milder climates will undergo one more physical transformation: their ovaries will turn into testes, and they'll develop a secondary sex characteristic called "nuptial pads" (a mucous gland which helps the frog grab a female during mating).

There's also a *third* sex race of common frogs: ones which sometimes use chromosomal sex determination and sometimes don't.

It's unclear as to exactly why the same species of frog determines sex in multiple different ways. But it seems to be working out pretty well for them, since they're so plentiful in the wild.